Customize: provide a path to editing posts created in nav menus

There is currently a usability dead-end where there is no clear path to editing newly-created posts in nav menus. The initial proposal, which was patched in #34923, was to show users an info notification via the notifications API. We need to refine that approach or come up with another way to provide links to edit items once they're published. Note that edit links in the preview do not work.

Here is the flow that was proposed in #34923 (and also patched there, and that code could be reused here if we go with a similar approach):

Summarizing the discussion in the design chat, there are varying opinions on whether it's an improvement to be able to edit a post, but be taken outside of the customizer context, as opposed to not showing links at all. @helen pointed to the former behavior of the image editor linking to a separate admin page from the media modal as a parallel example.

One idea that might be worth exploring is to open a modal with an existing editor. @westonruter suggested the admin editor as a possible exploration, but the press this editor might be a viable option to explore there. We could also do a new-tab link to the admin as was previously proposed. Or we could leave this as-is without a path to editing until it's possible to edit them directly in the customizer.